1. Field of the Invention
The present invention relates generally to a modulation system and method which is to be incorporated as part of an AM stero radio station, and in particular, to an AM quadrature modulator having independent channel modulation.
2. Description of the Prior Art
Many types of AM stereo modulation systems are known in the art. These include quadrature modulation systems in which a first signal modulates a carrier signal, and a second signal modulates a carrier signal having a 90.degree. phase difference from the first carrier signal. The second, or quadrature, modulator is a suppressed carrier type, such that only the sidebands of the modulated signal remain. The quadrature modulated sidebands are added to the output signal of the first modulator to produce quadrature modulation.
For AM stereo transmission, the input of the first transmitter is the main (L+R) signal and the input of the second transmitter is the stereo (L-R) signal. This "pure quadrature" modulation is not compatible with current AM receivers.
A compatible quadrature modulation system, as described in Motorola Incorporated's "Introduction to Motorola C-Quam AM Stereo System" by Chris Payne, dated 1982, first generates pure quadrature modulation as described above, after which the quadrature signal is fed through a limiter which removes the amplitude information, i.e., inphase sidebands, and leaves only the quadrature phase information in the carrier signal. This quadrature phase shifted carrier is fed into a broadcast transmitter as the radio frequency input and the main (L +R) signal is fed to the broadcast transmitter as the audio input.
Other AM stereo quadrature modulators are disclosed in U.S. Pat. Nos. 4,401,853; 4,373,115; 4,324,952; 4,323,731; 4,236,042; and 4,225,751.
The above quadrature systems perform the summing and difference functions prior to modulation of the sum and difference signals. If the two modulators in those systems are not precisely balanced, cross talk occurs between the two channels. Thus, to adjust for best left-to-right channel separation requires a trade-off in right-to-left channel separation and to adjust for best right-to-left separation requires a trade-off in left-to-right separation.